Abstract
A dominant view in numerical cognition is that processing the quantity indicated by numbers (e.g. deciding the larger between two numbers such as ‘12.07’ or ‘15.02’) relies on the intraparietal regions (IPS) of the cerebral cortex. However, it remains unclear whether the IPS could play a more general role in numerical cognition, for example in (1) quantity processing even with non-numerical stimuli (e.g. choosing the larger of ‘bikini’ and ‘coat’); and/or (2) conceptual tasks involving numbers beyond those requiring quantity processing (e.g. attributing a summer date to either ‘12.07’ or ‘15.02’).In this study we applied fMRI-guided TMS to the left and right IPS, while independently manipulating stimulus and task. Our results showed that IPS involvement in numerical cognition is neither stimulus-specific nor specific for conceptual tasks. Thus, quantity judgments with numerical and non-numerical stimuli were equally affected by IPS-TMS, as well as a number conceptual task not requiring quantity comparisons. However, IPS-TMS showed no impairment for perceptual decisions on numbers without any conceptual processing (i.e. colour judgment), nor for conceptual decisions that did not involve quantity or number stimuli (e.g. summer object: ‘bikini’ or ‘coat’?). These results are consistent with proposals that the parietal areas are engaged in the conceptual representation of numbers but they challenge the most common view that number processing is so automatic that the simple presentation of numbers activates the IPS and a sense of magnitude. Rather, our results show that the IPS is only necessary when conceptual operations need to be explicitly oriented to numerical concepts.
Highlights
Neuroimaging studies have shown that the parietal regions, especially those around the intraparietal sulcus (IPS)1 are reliably activated when processing number quantities, for instance when deciding on the larger of two numbers (e.g. Dehaene et al, 2003; Nieder, 2005)
transcranial magnetic stimulation (TMS) did not affect the participants' accuracy in any of the tasks [overall 4.6% of errors, no significant difference between TMS and sham across tasks and stimuli, p N 0.1]. This is consistent with similar paradigms or tasks used in posterior parietal cortex TMS showing that tasks performed at high levels of accuracy are likely to result in reaction times (RTs) deficit rather than increase in error rate following TMS (Alexander et al, 2005; Ashbridge et al, 1997; Cappelletti et al, 2007)
This study investigated the role of the left and right IPS regions in processing quantity with numerical and non-numerical stimuli, and in conceptual tasks with numbers requiring or not quantity manipulation
Summary
Neuroimaging studies have shown that the parietal regions, especially those around the intraparietal sulcus (IPS) are reliably activated when processing number quantities, for instance when deciding on the larger of two numbers (e.g. Dehaene et al, 2003; Nieder, 2005). Cipolotti et al, 1991; Dehaene and Cohen, 1991; Lemer et al, 2003; Polk et al, 2001), and TMS studies have reported impaired Despite this converging evidence, it is still unknown whether the IPS is critical for: (1) quantity processing irrespective of the stimuli used, i.e. numerical or non-numerical; and (2) other conceptual processing of numbers that do not require quantity manipulation (e.g. attributing a summer date to either ‘12.07’ or ‘15.02’). A few studies have examined the stimulus-specific nature of quantitative processing These have proposed that quantitative judgments on different types of stimuli may either rely on distinct subregions within the parietal lobe, or that the parietal regions are involved in a generic process of comparison of various types of stimuli. The results of imaging studies examining this issue are mixed: while some of these studies showed common activation of the parietal areas in quantitative judgments irrespective of the stimuli used (e.g. Fias et al, 2003), others demonstrated that different stimuli are represented in distributed
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